ci: Upgrade nightly toolchain

crates/ruma-html/build.rs

@@ -0,0 +1,6 @@
+fn main() {
+    // Prevent unnecessary rerunning of this build script
+    println!("cargo:rerun-if-changed=build.rs");
+    // Prevent nightly CI from erroring on docsrs attributes
+    println!("cargo:rustc-check-cfg=cfg(docsrs)");
+}